In the related arts, in many cases, a threaded body is used to fasten objects to be fastened together. The threaded body is provided in a structure in which a male screw body is screwed with a female screw body. The male screw body may include spiral grooves on a pillar-shaped outer circumference, and the female screw body may include spiral grooves on an inner circumference of a tubular member.
The threaded body is used in a way that the male screw body is inserted into a hole formed on a member to be fastened, and screwed with the female screw body. Then, the objects are fastened together through a head part of the male screw body and the tubular member of the female screw body. For easy use of the threaded body for fastening, there was designed a threaded body including a so-called bolt having a hexagonal prism-shaped head part at one end of a male screw body, and a nut having an outer circumferential surface provided in a shape of a hexagonal prism-shaped column. In addition, a so-called small screw having a head part with a metal slit (minus-shaped slot or cross-shaped slot) on a cross section, rather than a hexagonal shape, is widely used.
Further, a washer is inserted around the male screw body when fastening the threaded body. The washer prevents the objects to be fastened from buckling or damage when fastening, or conversely restricts loosening of the threaded body by aggressively and strongly pressing the objects to be fastened.
For example, according to “Japanese Industrial Standards (JIS) 1251 Spring Washer”, washers include a general ring-shaped “flat washer”, a “tongue washer” having a protrusion extending in a radial direction on an outer or inner circumference and configured to engage a member to be fastened or a threaded body to prevent loosening, a “nail washer” having nails bent in an axial direction to engage an object to be fastened, a “spring washer” configured to prevent loosening of a threaded body through elastic deformation, and a “toothed washer” having teeth around to lock into a fastening surface.